Classifications

• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/24—Coupling light guides
  • G02B6/42—Coupling light guides with opto-electronic elements
  • G02B6/4298—Coupling light guides with opto-electronic elements coupling with non-coherent light sources and/or radiation detectors, e.g. lamps, incandescent bulbs, scintillation chambers
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
  • G02B27/48—Laser speckle optics
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/04—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings formed by bundles of fibres
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
  • G01N21/47—Scattering, i.e. diffuse reflection
  • G01N21/4788—Diffraction
  • G01N2021/479—Speckle
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/84—Systems specially adapted for particular applications
  • G01N21/88—Investigating the presence of flaws or contamination
  • G01N21/95—Investigating the presence of flaws or contamination characterised by the material or shape of the object to be examined
  • G01N21/9501—Semiconductor wafers
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/24—Coupling light guides
  • G02B6/42—Coupling light guides with opto-electronic elements
  • G02B6/4201—Packages, e.g. shape, construction, internal or external details
  • G02B6/4204—Packages, e.g. shape, construction, internal or external details the coupling comprising intermediate optical elements, e.g. lenses, holograms
  • G02B6/4206—Optical features

Definitions

• the present invention relates to the field of fiber optical systems for illumination of objects to be imaged, and especially to ways of reducing speckle effects arising from the coherence of the illumination source used.
• the laser beam itself is generally not uniform. Using the laser beam directly as a light source creates non-uniform illumination.
• Spatial coherence which is the phase relation between each spatial point in the laser beam spot. This allows different points in the spot to interact with each other in a destructive or constructive manner when the spot is illuminating a cyclic pattern or a rough surface. This quality depends mainly on the mode of the beam. For instance, in the basic mode (TEMoo) the spatial coherence is defined by the Gaussian profile of the beam.
• Temporal coherence which is a measure of the time or the transit distance (the time multiplied by the speed of light in the medium concerned) over which the phase of the beam can be defined. This parameter depends on the type of laser and its spectral bandwidth. Thus, for instance, for the second harmonic of a Nd:YAG laser at 532 nm, the coherence length is about 8 mm in free space.
• the present invention seeks to provide a new fiber optical illumination delivery system, which is effective in reducing the speckle effects arising from source coherence.
• the system preferably utilizes either a single bundle of optical fibers, or serial bundles of optical fibers, according to the various preferred embodiments of the present invention.
• the single bundle embodiment differs from prior art systems in that the differences in optical lengths between different fibers of the bundle is preferably made to be equal to or more preferably less than the coherence length of the source illumination.
• This preferred embodiment enables construction of an illumination system delivering a higher level of illumination, but without greatly affecting the coherence breaking abilities of the system, thus enabling a generally more applicable and cost- effective system to be constructed.
• the serial bundle embodiment differs from prior art systems in that in the bundle comprising the fibers, where in the prior art systems, the differences in lengths of the fibers therein is made equal to the overall difference in length between the shortest and the longest fibers in the other bundle, according to a preferred embodiment of this invention, there are arranged groups of fibers of the same length, and it is the difference in lengths of these groups which is made equal to, or even more preferably, less than the overall difference in length between the shortest and the longest fibers in the other bundle.
• This preferred embodiment also enables construction of an illumination system delivering a higher level of illumination, but without greatly affecting the coherence breaking abilities of the system, thus enabling a generally more applicable system to be constructed.
• an optical system for reducing the coherence of a beam for illumination of an object comprising a source of at least partially coherent illumination, at least part of which has a characteristic coherence length, and at least one fiber optics bundle comprising a plurality of optical fibers, at least some of which have differing optical lengths, at least some of the fibers of differing optical length having differences in optical lengths therebetween which are less than the characteristic coherence length.
• the source of at least partially coherent illumination may preferably be a laser source, and the coherent illurnination may have spatial coherence or temporal coherence or both.
• the plurality of optical fibers in the at least one fiber optics bundle are preferably randomly ordered.
• a diffusing element may be used for spatial mixing of the beam.
• the optical system may also comprise an optical element positioned such that it is operative to direct the illumination from any point of the beam into essentially each of the plurality of fibers.
• the differences in optical lengths being less than the characteristic coherence length results in a bundle having reduced transmission losses.
• the illumination beam comprises pulses having a characteristic length
• the bundle is operative to stretch the length of the pulses.
• an optical system for reducing the coherence of a beam for illumination of an object comprising a source of at least partially coherent illumination, at least part of the illumination having a characteristic coherence length, a first fiber optics bundle comprising a plurality of optical fibers, at least some of which have differing optical lengths, at least some of the fibers of differing optical length having differences in optical lengths therebetween which are less than the characteristic coherence length, and a second fiber optics bundle disposed serially with the first bundle, comprising a plurality of groups of optical fibers, each group of fibers comprising fibers of essentially the same length, and wherein at least some of the group of fibers have differing optical lengths, at least some of the groups of fibers having differences in optical lengths therebetween which are at least equal to the sum of all of the optical length differences of the fibers in the first bundle.
• each of the groups may have essentially the same number of fibers, or alternatively and preferably, the number of fibers in each of the groups may increase according to the optical length of the group, and even more preferably, the number of fibers in each group may generally be proportional to the length of the group.
• the bundles may be arranged serially such that the beam for illumination of the object is initially incident on the first bundle or alternatively and preferably, the beam for illumination of the object is initially incident on the second bundle.
• an optical element is positioned between the bundles such that it is operative to direct illumination from any point of the output of the first bundle onto essentially each point of the input of the second bundle.
• the source of at least partially coherent illumination may preferably be a laser source, and the coherent illumination may have spatial coherence or temporal coherence or both.
• the plurality of optical fibers in the at least one fiber optics bundle are preferably randomly ordered.
• a diffusing element may be used for spatial mixing of the beam.
• a method of reducing the transmission loss in a fiber optical bundle for reducing the coherence of light transmitted therethrough, at least part of which light has a characteristic coherence length comprising the steps of providing at least one fiber optical bundle comprising a plurality of optical fibers, at least some of which have differing optical lengths, and arranging the lengths of the plurality of optical fibers such that at least some of the fibers of differing optical lengths have differences in optical length therebetween generally less than the characteristic coherence length.
• Fig. 1 is a schematic illustration of a bright field object inspection system, utilizing a laser source . and a fiber optical delivery bundle, constructed and operative according to a preferred embodiment of the present invention
• Fig. 2 is a schematic drawing of a fiber optical delivery bundle, according to a preferred embodiment of the present invention, such as that used in Fig. l;
• FIG. 3A to 3E schematically show various preferred embodiments of fiber bundle applications, according to further preferred embodiments of the present invention.
• Fig. 3 A is a graphical illustration of the transmission and the coherence reduction factor of a single fiber optical bundle, such as that shown in the embodiment of Fig. 1, as a function of fiber optical length difference divided by the coherence length of the source;
• Fig. 3B is a schematic illustration of a double bundle fiber optical illumination system, according to a preferred embodiment of the present invention.
• Figs. 3C and 3D respectively illustrate schematically two embodiments of a first bundle of a double bundle illumination ' system, such as that of Fig. 3B, according to another preferred embodiment of the present invention, in which the bundle is made up of groups of fibers of the same length;
• Fig. 3E is a schematic drawing of the second bundle of fibers of the preferred embodiment of Fig. 3B, in which each of the fibers is of a different optical length, the optical lengths preferably differing by the coherence length of the light source or less.
• Fig. 1 is an overall schematic side view of the complete illumination system of the defect detection apparatus, according to one preferred embodiment of the present invention.
• three alternative modes of illumination are provided: Bright Field (BF), Side-illuminated Dark Field (DF) and Orthogonal or Obscured Reflectance Dark Field (ODF).
• BF Bright Field
• DF Side-illuminated Dark Field
• ODF Orthogonal or Obscured Reflectance Dark Field
• Each mode of illumination is used to detect different types of defects in different production process steps. For example in order to detect an embedded defect in a transparent layer, such as silicon oxide, BF illumination is preferred. In order to detect a small particle on a surface, DF illumination generally yields better results.
• Fig. 1 shows a bright field illuminating laser source 300 delivering its output beam 15 into an optical delivery fiber bundle 21, preferably by means of a laser to fiber coupler 150.
• This optical fiber bundle 21 is required for the dual purposes of providing uniform illumination on the sample and for coherence breaking of the laser illumination, as will be expounded further hereinbelow.
• a serial fiber bundle solution as will be shown hereinbelow, could just as readily have been used.
• the laser beam is imaged by means of illumination transfer lenses 301, 302, onto the objective lens in use 201, which is operative to focus the illumination onto the wafer plane 100 being inspected.
• Appropriate alternative objective lenses 201' can be swung into place on an objective revolver 200, as is known in the microscope arts.
• the illumination returned from the wafer is collected by the same objective lens 201, and is deflected from the illumination path by means of a beam splitter 202, towards a second beam splitter 500, from where it is reflected through the imaging lens 203, which images the light from the wafer onto the detector 206.
• the second beam splitter 500 is used to separate the light going to the imaging functionality from the light used in the auto-focus functionality, which is directed by means of the auto-focus imaging lens 501 to the auto-focus detector 502.
• a dark field side illumination source 231 is used to project the required illumination beam 221 onto the wafer 100.
• an alternative dark field illumination source 230 is used to project the required illumination beam 232 via the obscured reflectance mirror 240 onto the wafer 100 orthogonally from above.
• a repetitively pulsed laser source is preferably used in the illumination system of the present invention, though according to other preferred embodiments, CW laser illumination may also be used.
• the third harmonic of a Nd: YAG Laser output is preferably used.
• Speckle effects with CW lasers is comparatively easy to overcome, since it is possible to average the signal while varying the wave front.
• Several methods are described in the prior art for achieving this. When, however, the imaging process utilizes a single pulse for each acquired image, such a method becomes impossible to implement.
• the laser beam is thus divided into many parts, each part having no defined phase coherence with the other parts.
• each point in the field of view (FOV) on the sample is illuminated by all parts of the laser beam.
• Each part of the beam is coherent or partially coherent with itself and thus may contribute to the generation of speckle, or to other interference effects that create high contrast artifacts in the image. Since each part of the beam is not
• the total effect is averaged.
• the residual coherence effect depends on the number of beamlets used Since each beamlet is independent of the others, the interference effect is reduced by the square root of the number of beamlets, assuming that all beamlets have the same intensity contribution. Consequently ⁇ the greater the number of beamlets, the lower the level of appearance of coherence artifacts in the image.
• the laser beam is introduced into a fiber optics bundle, such as the fiber bundle 21 shown schematically in Fig. 1.
• the fibers in the bundle differ in length from each other by distances of the order of the laser coherence length in the fiber medium, or less.
• the number of fibers in the bundle dictates the contrast of the residual coherence effect in the image.
• the fiber bundle should preferably be illuminated uniformly.
• Each fiber in the bundle must carry more or less the same energy; otherwise averaging of the coherence effect will not be efficiently performed. Since the laser beam itself is not uniform and contains high and low spatial frequency components, the laser beam must be spatially mixed before introduction into the fiber.
• the full numerical aperture of the fiber should preferably be filled, since at the far end of the bundle, uniform angular distribution of intensity is required. These latter two requirements do not appear to be fulfilled in the prior art.
• Koehler illumination is generated, no arrangement is shown for spatially mixing the laser beam, nor is there described a specific method for ensuring that the incident light is directed such that the Numerical Aperture of each fiber is fully illuminated. Under the conditions shown, each fiber would illuminate randomly, resulting in non-uniform field stop plane intensity, which then would also result in non- uniform illumination at the object plane.
• the proposed array is made of N fiber-guides in which the length difference of any two fibers is greater than the coherence length of the light source.
• Such an arrangement would generally result in excessive differences, since it is the optical length difference and not the absolute length difference of any two fibers which needs to be greater than the coherence length of the light, according to the criteria chosen in the Dingel et al. article.
• the illumination system described in this prior art is for a transmissive imaging system.
• the laser beam 15 which can be either a parallel beam or slightly convergent, or slightly divergent impinges onto a diffusing element 16, which, according to alternative and preferred embodiments, can be a regular diffuser, a holographic diffuser (such as an MEMS) or a micro-lens array having a numerical aperture that spreads the incident light at the required angles.
• the diffused beam shown schematically in Fig.
• a focusing element 18 which can be either a single lens, or, in order to reduce aberrations, a multi-element lens.
• Rays diffused at different angles from those rays 17 shown in Fig. 2 are imaged onto different points of the end face 20 of the fiber optics bundle 21.
• Light from all of the included angles at which light is output from the diffuser is thus imaged by means of the focusing element 18 to cover the entire input aperture of the fiber bundle end face 20.
• the beam traverses the fiber bundle 21 and is output at the opposite end face 29 of the fibers at the output connector 22.
• the diffusing element 16 is preferably positioned at the left focal plane of the focusing element 18, and the end face 20 of the fiber 21 , at the right focal plane of the focusing element.
• the half angle of the diffusing element, and the focal length f, of focusing element are computed as follows:
• Fig. 3 A is a graphical illustration of the outcome of the above-described trade-off in fiber length difference between transmission and coherence breaking efficiency.
• the results shown in Fig. 3 A are for a bundle containing 40,000 fibers, and for a fiber having a transmission loss in the UV. of the order of O.ldb/m.
• the two ordinates separately show the bundle transmission and the coherence reduction factor as a function of Delta/Lc, where each fiber differs in length by Delta mm., and the coherence length of the source is LQ.
• the transmission is measured relative to a bundle having uniform fiber lengths equal to the length of the shortest fiber in the variable fiber length bundle.
• the value of Lc for the example shown is 5mm.
• the coherence reduction factor is approximately 90, compared to the maximum theoretical 200.
• the coherence reduction factor falls short of its theoretical value because the increasing insertion loss of each successive fiber means that the intensity contribution of each separate fiber to the total output is not equal, and the coherence breaking effect is thus reduced.
• the coherence reduction factor is reduced to approximately 85, which is only a 6% reduction, while the transmission is increased to approximately 0.45, which is over a 110% increase.
• an illumination delivery fiber bundle operative for breaking the coherence of light transmitted therethrough, in which the differences in lengths of the fibers in the bundle are less than the coherence length of the source.
• Fig. 3B is a schematic illustration of an optical arrangement for achieving this result, wherein a second bundle is provided serially with the first bundle of Fig. 2.
• a second bundle is provided serially with the first bundle of Fig. 2.
• three exemplary rays 23 propagating at the same angle from different locations in the end face 20 are shown being imaged onto the end face 26 of the fibers at the terminal connector 25 of the second fiber optics bundle 27 by means of a focusing element 24, which can be either a single lens, or a multi-element lens.
• the beam is output from the second fiber bundle 27 at the far end face 26 of the fibers at the output connector 28.
• the diameter of the first bundle 21 be the same as the diameter of the second bundle 27, as shown in the preferred embodiment of Fig. 3B. If the first bundle has a smaller diameter, a diffuser is required at its end to increase the angular distribution of light from the end, in order to fill the input of the second bundle.
• the fibers in both bundles are described as having different lengths, and the difference in length ⁇ L between any two fibers in one bundle is preferably selected to be greater than the coherence length of the light source.
• the difference in length between any pair in the other bundle is described as being preferably larger than the difference in length between the shortest and the longest fiber in the first mentioned bundle.
• the difference in length between any pair of fibers in the first bundle is described as being preferably larger than the difference in length between the shortest and the longest fiber in the other bundle.
• the difference in length between any pair of fibers of that other bundle is described as being greater than the coherence length of the light source, such that the difference in length between the shortest and the longest fiber in the other bundle is thus greater than the coherence length of the light source times the number of fibers in the other bundle.
• the typical coherence lengths generated by lasers used for such applications are of the order of up to a few millimeters. Consequently, according to the criteria of this prior art, there is an appreciable difference in length between the fibers of the first bundle.
• the differences between the lengths of the fibers in the second bundle should preferably be more than the coherence length in order to generate efficient coherence breaking in such a bundle, and on the other hand, the larger the difference in lengths between the fibers anywhere in the double bundle embodiment, the more the coherence breaking in the second fiber is degraded because of lack of unity of intensity.
• the difference in length ⁇ L between any two fibers in one bundle is preferably selected to be greater than the coherence length of the light source.
• This preferred difference in length is longer than the optical path length in the fiber by a factor N, where N is the refractive index of the core material, such that this method proposes use of a longer length difference between fibers than is dictated by optical considerations, even before any incentive to reduce fiber length differences, as discussed hereinabove.
• Figs. 3C and 3D respectively illustrate schematically the two bundles of a double fiber bundle delivery system, constructed and operative according to another preferred embodiment of the present invention.
• This embodiment is operative to diminish the above-described disadvantages of the prior art double fiber bundle delivery system.
• the fiber bundle shown in Fig. 3C is regarded as the input bundle, denoted 21 in the embodiment of Fig. 3B
• the fiber bundle shown in Fig. 3D is regarded as the output bundle, denoted 27 in Fig. 3B, though it is to be understood that this embodiment is equally operable with the fibers in either order, if the correct matching components are provided.
• every fiber should optimally be of a different optical length by the sum of all of the optical length differences of the fibers in the second bundle.
• equal optical length fibers should ideally be used, but this would generate no coherence breaking in the first bundle.
• the fibers within each group provide an element of uniformity to the beamlets output from the first bundle, while the difference in optical lengths between the groups provides the coherence breaking properties of the light from the different groups.
• the correct trade-off between these two effects is able to compensate to a large extent for the reduction in efficiency from the coherence breaking effect that would be obtained if all the fibers were of different optical lengths, but were also loss free, such that the intensity change effect was not a factor.
• the extent of the compensation between these two effects is a function of the attenuation per unit length of the fiber used.
• Fig. 3D is a schematic drawing of a bundle of fibers, according to yet another preferred embodiment of the present invention, similar to that shown in Fig. 3C, but in which the number of fibers in each group is increased according to the length of the group. Even more preferably, the number of fibers in each group is made generally proportional to the length of the group. In this way, the increased insertion loss arising in a group because of the additional fiber length in the group is offset by the increase in the number of fibers in that group.
• a further advantage in the use of groups of fibers, according to this embodiment of the present invention, is that the redundancy effect of a large number of fibers operating in parallel has the effect of smoothing out any production differences which inevitably arise between supposedly identical fibers, both in optical properties and in targeted cleaved length.
• each of the fibers is preferably of a different optical length, the optical lengths preferably differing by the coherence length of the light source or less. Since the total overall difference in optical lengths of the fibers in the second bundle is determinative in fixing the difference in the optical lengths between the different groups of fibers in the first bundle, and as mentioned above, there is an advantage in keeping the path differences between fibers as short as possible to minimize intensity changes between fibers or fiber groups, there is an additional advantage to the first bundle parameters if the optical lengths of the fibers in the second bundle differ by as little as possible.
• a system comprising only one bundle of the type containing the groups of fibers, whether that bundle is positioned in front of or after the bundle containing the single fibers.
• a plurality of bundles with groups of fibers can be used, instead of a single such bundle, such that the illumination system comprises a series of bundles of fibers, with the groups of fibers and the fibers respectively optimally arranged for good coherence breaking properties and minimal transmission losses, as expounded hereinabove.
• Fig. 3E which has k fibers, where k is preferably of the order of 1000.
• the length of a first fiber is L, where L is preferably of the order of 1 meter.
• a second fiber is longer than the first by LJN, where L c is the coherence length of the laser source, typically 6mm, and N is the fiber core refractive index, generally of the order of 1.5, such that the fiber length difference is of the order of 4 mm.
• a third fiber is longer than the second also by LJN, and so on. The sum of all k length differences is thus k x L c /N , which amounts to the order of 4 meters for this preferred example.
• the first bundle has a number n of groups of fibers, where n is preferably 10 to 20.
• Each group contains m, preferably 20 to 50, fibers of equal length and equal optical path length.
• the length difference between each of the groups is equal to or greater than the sum of all of the length differences of the second bundle, which, in this preferred example, amounts to approximately 4 meters, as obtained above. From these numerical examples, the reason for limiting differences in fiber lengths to limit transmission loss changes becomes evident.
• the above-described embodiments of the present invention for achieving beam coherence breaking also result in a solution for a problem related to the use of short pulsed lasers in such illumination systems.
• Such short laser pulses which can typically be as short as only a few nanoseconds, may have a peak power density so high that the focussed beam may cause damage to the wafer under inspection.
• a common method used to decrease the peak power of a short laser pulse is to stretch the pulse, such that the pulse energy is expended over a longer time, and hence has a lower peak power.
• Such pulse stretching can be performed by transmitting the pulse in parallel down several paths of different optical path length, and recombining after transit. This is the situation which exists with the assembly of variable length fibers in the bundles shown in the embodiments of Figs. 2 and 3 A - 3E of the present invention, such that the fiber bundles of the present invention are also effective in pulse stretching applications.
• the above mentioned numerical example will be used.
• a total length difference of 80 meters is generated.
• the time of flight of light in the medium of the fiber, having a refractive index of 1.5, is approximately 5 nsec/meter.
• the total time of flight difference for an 80 meter bundle is approximately 400 nsec.
• the effect of the bundle is thus to generate pulse stretching from the typically few nanosecond pulse lengths emitted by the laser, to about two orders of magnitude longer, with the concomitant reduction in potential beam damage.
• some or all of the optical path differences between fibers is less than the beam coherence length.
• a particular feature of a preferred embodiment of the present invention is that the system includes a second fiber optic bundle, within which the optical path length difference between each pair of fibers is less than or equal to the coherence length of the light beam being employed by the system. It is appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described hereinabove. Rather the scope of the present invention includes both combinations and subcombinations of various features described hereinabove as well as variations and modifications thereto which would occur to a person of skill in the art upon reading the above description and which are not in the prior art.

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